Subsurface safety valves are used in wells to close them off in the event of an uncontrolled condition to ensure the safety of surface personnel and prevent property damage and pollution. Typically these valves comprise a flapper, which is the closure element and is pivotally mounted to rotate 90 degrees between an open and a closed position. A hollow tube called a flow tube is actuated downwardly against the flapper to rotate it to a position behind the tube and off its seat. This is described as the open position. When the flow tube is retracted the flapper is urged by a spring mounted to its pivot rod to rotate to the closed position against a similarly shaped seat.
The flow tube is operated by a hydraulic control system that includes a control line from the surface to one side of a piston. Increasing pressure in the control line moves the piston in one direction and shifts the flow tube with it. This movement occurs against a closure spring that is generally sized to offset the hydrostatic pressure in the control line, friction losses on the piston seals and the weight of the components to be moved in an opposite direction to shift the flow tube up and away from the flapper so that the flapper can swing shut.
Normally, it is desirable to have the flapper go to a closed position in the event of failure modes in the hydraulic control system and during normal operation on loss or removal of control line pressure. The need to meet normal and failure mode requirements in a tubing pressure insensitive control system, particularly in a deep set safety valve application, has presented a challenge in the past. The results represent a variety of approaches that have added complexity to the design by including features to ensure the fail safe position is obtained regardless of which seals or connections fail. Some of these systems have overlays of pilot pistons and several pressurized gas reservoirs while others require multiple control lines from the surface in part to offset the pressure from control line hydrostatic pressure. Some recent examples of these efforts can be seen in U.S. Pat. No. 6,427,778 and 6,109,351.
Despite these efforts a tubing pressure insensitive control system for deep set safety valves that had greater simplicity, enhanced reliability and lower production cost remained a goal to be accomplished. The present invention offers a system that features a single control line that acts on a piston that extends through spaced blocks so that it is substantially in pressure balance from tubing pressure. Each block has a tubing pressure seal while the piston carries a control line pressure seal in the upper block. A passage between the seals in the upper block extends preferably through the piston to a reservoir holding a compressible gas preferably near atmospheric pressure. The movement of the piston compresses the fluid in the reservoir and compresses a closure spring acting on the flow tube. Optionally, a spring or/and an equivalent can act on the piston directly to move the flow tube to close the valve. A redundant system can be provided so that when the primary system fails and is pressure equalized because of such failure, access into a redundant system from the same or separate control line can be obtained for continued operation of the valve.
Those skilled in the art will better appreciate the details of the invention from the description of the preferred embodiment and the drawings that appear below while recognizing that the full scope of the invention is indicated by the claims.